While vaccination protocols have been some of the great medical achievements in the last century, there are still conditions where an effective immune response has been difficult to generate. For example, human tumor immunotherapy has met with only limited success. Among the reasons for this have been the limited availability of tumor-associated antigens, and an inability to deliver such antigens in a manner that renders them immunogenic. Recent insights into the role of dendritic cells (DC) as the pivotal antigen presenting cell for initiation of immune responses may provide the basis for more effective immune responses, particularly where conventional vaccination is inadequate.
The events whereby cells fragment antigens into peptides, and then present these peptides in association with products of the major histocompatibility complex, (MHC) are termed xe2x80x9cantigen presentationxe2x80x9d. The MHC is a region of highly polymorphic genes whose products are expressed on the surfaces of a variety of cells. T cells recognize foreign antigens bound to only one specific class I or class II MHC molecule. The patterns of antigen association with class I or class II MHC molecules determine which T cells are stimulated.
T cells do not effectively respond to antigen unless the antigen is processed and presented to them by the appropriate antigen presenting cells (APC). The two major classes of antigen presenting cells are DC and macrophages. DC are uniquely capable of processing and presenting antigens to naive T cells. The efficacy of DC in antigen presentation is widely acknowledged, but the clinical use of these cells is hampered by the fact that there are very few in any given organ. In human blood, for example, about 1% of the white cells are DC. While DC can process foreign antigens into peptides that immunologically active T cells can recognize, the low numbers of DC makes their therapeutic use very difficult.
In recent years, the life cycle of DC has been elucidated. DC precursors migrate from bone marrow and circulate in the blood to specific sites in the body where they mature. This trafficking is directed by expression of chemokine receptors and adhesion molecules. Tissue resident DC include Langerhans cells in skin, hepatic DC in the portal triads, mucosal DC and lung DC. Upon exposure to antigen and activation signals, the DC are activated, and leave tissues to migrate via the afferent lymphatics to the T cell rich paracortex of the draining lymph nodes. The activated DC then secrete chemokines and cytokines involved in T cell homing and activation, and present processed antigen to T cells.
Mature DC have a distinct morphology characterized by the presence of numerous membrane processes. These processes can take the form of dendrites, pseudopods or veils. DC are also characterized by the cell surface expression of large amounts of class II MHC antigens and the absence of lineage markers, including CD14 (monocyte), CD3 (T cell), CD19, 20, 24 (B cell), CD56 (natural killer), and CD66b (granulocyte). DC express a variety of adhesion and co-stimulatory molecules, e.g. CD80 and CD86, and molecules that regulate co-stimulation, such as CD40. The phenotype of DC varies with the stage of maturation and activation, where expression of adhesion molecules, MHC antigens and co-stimulatory molecules increases with maturation. Antibodies that preferentially stain mature DC include anti-CD83 and CMRF-44.
While methods have been described for the in vitro manipulation of DC in order to enhance their immunologic function, such techniques can be very expensive and labor intensive. The ability to enhance DC antigen presentation in vivo (i.e. without in vitro culture) would be of great clinical and experimental benefit.
Relevant Literature
Administration of Flt3 ligand to mice in vivo results in preferential mobilization or release of DC precursors from the bone marrow to the periphery and into lymphoid organs, and can increase the number of circulating DC 10-30 fold (Maraskovsky et al. (1996) J. Exp. Med. 184:1953-1962). It has been suggested that these cells can then be removed for ex vivo manipulation and priming. Pulendran et al. (1997) J. Immunol. 159:2222-2231 describe the expansion of DC in animals treated with FL.
Hsu et al. (1996) Nat. Med. 2:52-58 describe vaccination of patients with B-cell lymphoma using autologous DC that had been removed from the patient, pulsed with antigen, and reinfused as an autologous vaccine. Young and Inaba (1996) J. Exp. Med. 183:7-11 describe the use of DC are adjuvants for class I MHC-restricted antitumor immunity.
U.S. Pat. No. 5,994,126, Steinman et a., issued Nov. 30, 1999 describes method for in vitro proliferation of DC precursors and their use to produce immunogens. U.S. Pat. No. 5,976,546, Laus et al., issued Nov. 2, 1999 describes immunostimulatory compositions.
Methods are provided for enhancing the immunogenicity of an antigen by increasing the specific antigen presenting function of DC in a mammalian host. Prior to the immunization with an antigen, the host is treated with a DC mobilization agent, e.g. Flt-3 ligand, GM-CSF, G-CSF/FIt3L fusion protein, etc. This treatment effectively increases the number of circulating DC precursors. The host is then given a local, e.g. sub-cutaneous, intramuscular, etc., injection of antigen in combination with a DC activating agent, e.g. immunostimulatory DNA sequences, IL-1, alpha interferon, LPS, endotoxin, CD40L, poly IC, etc. The activation step promotes recruitment and maturation of the DC, along with antigen-specific activation and migration from the tissues to lymphoid organs. These DC then effectively interact with, and present processed antigen to, T cells that are then able to respond to the antigen. The methods of the invention are particularly useful in situations where the host response to the antigen is sub-optimal, for example in conditions of chronic infection, a lack of immune response to tumor antigens, and the like. In one aspect of the invention, the antigen is a tumor antigen, and is used to enhance the host immune response to tumor cells present in the body.